High creep strength low alloy steel



Patented Mar. 4, 1941 UNITED STATES HIGH CREEP STRENGTH LOW ALLQY STEELHarry L. Frevert, Philadelphia, Pa., assignor to The Midvale Company,Philadelphia, Pa., a cor poration of Delaware No Drawing.

Application November 14, 1939,

Serial No. 304,329

4 Claims.

There are certain articles of manufacture, now made of steel, alloyed ornot with one or more of the common steel-alloying elements, and which inuse are subjected to high temperatures, commonly within the range of800-1100 E, which it is highly desirable should have the qualities ofhigh creep strength and freedom from brittleness. Creep strength" may beroughly defined as that quality enabling the article to resistcontinuous deformation under constant load at high temperature. It isalso essential that these qualities should be maintained for a longperiod of time. It is well recognized that these articles subjected tosuch high temperatures, and particularly those which normally mustfunction at such high temperatures continuously for prolonged periods oftime, do not exhibit these qualities to a satisfactory degree, unlessmade of expensive, highly alloyed compositions.

The development of an inexpensive forging steel that will resist creepat temperatures practically up to incandescence is the object of my Iinvention. The problem presents unusual difficulties in view of the longtime required experimentally to determine whether or not givencompositions are durable with respect to the qualities sought, since thebehavior of different compositions, found on long time tests to retainthe desired qualities for markedlydififerent periods of time, do not, onshort time tests, exhibit any variance sufiicient to predicate therefromthe length of time during which such qualities may be expected to bemaintained.

In the manufacture of many such articles the use of straight low carbonsteel has been, found to be open to fewer objections than most of thecommercial alloy steels. Commercial alloy steels generally contain oneor more of the common alloying elements in considerable proportions,such proportions being often of value and importance in securing otherqualities than those above mentioned.

In view of the lack of superiority of commercial alloy steels over lowcarbon steel, in my investigation of the problem it occurred to me todetermine the eiiect of the addition of very small amounts of differentalloying elements. The result of my experimental work, which hasnecessistance.

the reverse to that exhibited by their addition in lower percentage.With respect to certain other elements it was found that increase of theamount added beyond a limited range does not improve the steel inrespect to this quality, or does not improve it to any material extent,and that such increase is open to objections of so serious a nature asnot to warrant the-use of the elements beyond the low percentage whichapproaches the optimum for the attainment of the best results. Theelements that have been found to give cheaply the results sought, whenall are added and when each is added in a certain small proportion, aremanganese, silicon, chromium and molybdenum. The addition of increasedproportions of these elements has been found to be not only ineffective,but even to impair the qualities produced by the addition of theseelements in the amounts specified.

Metallurgists are of course quite familiar with alloy steels containingthe particular combination of elements above specified, but the art isdevoid of any discovery that this particular combination of elements,each added in very small percentage within a particular narrow range, isessential to.

secure the particular qualities mentioned in the forepart of thisspecification without sacrifice of other necessary qualities.

Considering in order the alloys specified, manganese is added in anamount which is not uncommon, namely between .50 and 1.25%. additionsomewhat beyond the maximum specified does not decrease creep strength,but a larger amount makes the metal more difiicult to forge in largesizes and more sensitive to hardening under normal cooling conditionsand causes the highly objectionable condition known in the art as bandedferrite. It is desirable not to add more than about one per cent.

The addition of silicon is of some importance but preferably it shouldnot be present in a proportion above .4%. With a percentage of siliconbeyond .4%, the creep resistance decreases at an accelerating rate.

The optimum proportion of chromium is about .6%, but within the range of.5 to 1% its effect in increasing creep resistance is pronounced.Increasing the percentage of chromium beyondthe maximum specified hasthe opposite effect.

The addition of as little as .15% molybdenum has a substantial effect inincreasing creep recreases, the creep resistance also increases,although at a progressively decreasing rate. If molybdenum be addedbeyond 375% there is no increase of creep resistance commensurate withthe added cost and the steel becomes more difficult to work.

The total of the specified four added alloying elements should nottherefore exceed 3.4% and As the amount of molybdenum ln-.

It should be noted that the composition approaches more nearly that ofcarbon steel than do most alloy steels, although its improvement overcarbon steel is striking. j

As indicating the pronounced effect of adding chromium and molybdenum tocarbon steel containing manganese and silicon within the ranges abovespecified, the followingo'omparison of thecreep values of differentcompositions were obtamed: 1

Load producing 1%ll00,000-hrs creeprate Mn Si Cr Mo 800 F. 000 F. 1000 11100F (o) .20 .65 .55 11,000 0,500 3,000 1,820 (b) .24 1.05 .25 12,0004,500 (c) .24 1.25 .18 25,200 14,000 .1 (:0- .25 .05 .20 .02 .20 44.50025,500 0,000 2 .20 .50 .10 .45 .15 20,000 12,000 0,400 3,000

The results obtained on experimental composition v(d) were confirmedby athree-ton heat in which the composition (f)'-C .26, Mn .88, Si .17, Cr.60 and Mo .20-was nearly identical with composition ((1)4 Composition(a) does not meet requirements. Nor does composition (b), even thoughthe percentage of manganese is increased to about what has been found tobe the probably most effective proportion in the complete composition.When, however, the percentage of manganese is increased to about themaximum, and a very small proportion of molybdenum is added, apronounced improvement results, as is illustrated by composition (0).When to this composition is added a verysmall percentage of chromium asin composition (d) then the improvement is quite remarkable. However, asshown in composition (e), a reduction in manganese, chromium andmolybdenum produces a composition inferior to composition (0) whichcontains no chromium. This indicates that nearly twice the percentage ofmolybdenum will not compensate for low manganese and low chromium.

- It was also found, in outs de tests. by prospective users ofcompositions-essentially the same as (d) and (I). that after load ng at102 Fjfor 2200 hrs. the metal or composition has not undergone anydeterioration. its elastic limit, ductility and Charpy impact valueshaving as a matter of fact ncreased. Tests before and after loading at1022 F. for 2200 hrs. follow:

Charo T. 8. E. L. m lbs.

Elong.

Before testing at 1022 F 000 m After testing at 1022 F was waco

coaoo steels otherwise useful for high temperature service it does notundergo embrittlement.

Tests to determine the load required to fracture this material over longperiods of time produced the following results:

5 Load to produce Temperature, F. rup in 100,000hrs. P. S. I.

Fractures produced in rupturing bars incident to determining, byextrapolation, the above figures all showed ductile transcrystallinefractures as opposed to the brittle, intercrystalline fracture of steelswhich fail suddenly and without warning at such temperatures. Thesetests were made on material simply air quenched from 1550 F. and drawnat 1200 F., which treatment confers a fine microstructure without freeferrite.

Theonly element not specified-that I have discovered to have abeneficial efiect in attaining the object of theinvention is tungsten,which is much more expensive than molybdenum andto produce the sameeffect as molybdenum must be added in much larger proportion. Inspecifying, in the claims, after enumerating the elements abovedescribed, that the balance of the composition is substantially iron, itis not intended to exclude the addition of an inconsiderable amount ofany other element that is inefi'ective to materially reduce theefiiciency of the elements specified.

It will be noted that the total percentages of alloying elements otherthan the carbon in compositions (d) and (f) are respectively 1.9? and1.85%. The total percentage of these four ele- 'ments' should not exceedabout 3.4% or be less than 1.25% and preferably should be between 1.5and. 2.5%. In all cases the percentage of either manganese or chromiumshould not be much less than, and should usually exceed, that of themolybdenum; and the percentage of silicon should generally be less thanthat of any of the other elements except carbon, and always less thanthat of the manganese or the chromium.

For many purposes the composition has slight advantage over carbon steelof equivalent strength, but for many other uses, the creep resistance,the maintenance of that resistance with prolonged use, and freedom fromcold embrittlement after prolonged high temperature exposure, thatcharacterize my improved. composition, are of the utmost importance andvalue. Among articles of manufacture for such use may be mentionedovens, autoclaves, and other reaction chambers and vessels, turbinespindles and'other turbine elements (wherein very slight creep? is aserious matter), heat exchangers, tubing and piping, bolts and nuts.

What I claim and desire to protect by Letters Patent is:

1. An alloy having a high and durable creep strength and freedom fromcold embrittlement after prolonged high temperature exposure conmium'isin excess of the percentage oi molybdenum and in excess of thepercentage of silicon, the balance of the composition beingsubstantially all iron.

2. An alloy having a high and durable creep strength and freedom fromcold embrlttlement after prolonged high temperature exposure containingcarbon not over .5% and the addition elements manganese, silicon,chromium and molybdenum, the manganese being present in a proportionbetweem .65 and 1.5%, the chromium in a proportion between .5 and .9%,the

' silicon in a proportion between .15 and .40%,

and the molybdenum in a proportion between .15 and and having thefurther characteristic that the percentage of manganese is not less thanthe percentage of chromium plus the percentage of either of theremaining two addition elements and that the percentage of chromium isin excess of the percentage of molybdenum and in excess of thepercentage of silicon, the balance of the composition beingsubstantially all iron.

3. An alloy having a high and durable creep strength and freedom fromcold embrittlement after prolonged high temperature exposure containingcarbon not over .5% and the addition elements manganese, silicon,chromium and molybdenum, the manganese being present in a proportionbetween .85 and 1.1%, the chromium in a proportion between .5 and .9%,the silicon in a proportion between .15 and 3% and the molybdenum in aproportion between .15 and 4% and being further characterized by thefact that the percentage of manganese is not substantially less than thepercentage of chromium plus the percentage of the remaining two additionelements and that the percentage of chrois not substantially less thanapproximately the percentage of silicon and molybdenum combined, thebalance of the composition being substantially all iron.

4. An alloy steel having a high and durable creep strength and freedomfrom cold embrittlement after prolonged high temperature exposurecontaining carbon less than 50% and the additional elements manganese,silicon, chromium and molybdenum in a total percentage between 1.5and-3.4 per cent., the relative proportions of the addition elementsbeing so adjusted that the percentage of manganese is not less than thepercentage of chromium plus the percentage of molybdenum and that thepercentage of chromium substantially exceeds the percentage of eithermolybdenum or silicon, the balance of the composition beingsubstantially all iron.

HARRY L. FREVERT.

